Virtualization enables a single host machine with hardware and software support for virtualization to present an abstraction of the host, such that the underlying hardware of the host machine appears as one or more independently operating virtual machines. Each virtual machine may therefore function as a self-contained platform. Often, virtualization technology is used to allow multiple guest operating systems and/or other guest software to coexist and execute apparently simultaneously and apparently independently on multiple virtual machines while actually physically executing on the same hardware platform. A virtual machine may mimic the hardware of the host machine or alternatively present a different hardware abstraction altogether.
Virtualization systems may include a virtual machine monitor (VMM) which controls the host machine. The VMM provides guest software operating in a virtual machine with a set of resources (e.g., processors, memory, IO devices). The VMM may map some or all of the components of a physical host machine into the virtual machine, and may create fully virtual components, emulated in software in the VMM, which are included in the virtual machine (e.g., virtual IO devices). The VMM may thus be said to provide a “virtual bare machine” interface to guest software. The VMM uses facilities in a hardware virtualization architecture to provide services to a virtual machine and to provide protection from and between multiple virtual machines executing on the host machine. As guest software executes in a virtual machine, certain instructions executed by the guest software (e.g., instructions accessing peripheral devices) would normally directly access hardware, were the guest software executing directly on a hardware platform. In a virtualization system supported by a VMM, these instructions may cause a transition to the VMM, referred to herein as a virtual machine exit. The VMM handles these instructions in software in a manner suitable for the host machine hardware and host machine peripheral devices consistent with the virtual machines on which the guest software is executing. Similarly, certain interrupts and exceptions generated in the host machine may need to be intercepted and managed by the VMM or adapted for the guest software by the VMM before being passed on to the guest software for servicing. The VMM then transitions control to the guest software and the virtual machine resumes operation. The transition from the VMM to the guest software is referred to herein as a virtual machine entry.
As is known in the art, a page table is often used to provide a mapping from linear memory to physical memory in a typical processor based system. Page tables are generally memory-resident structures and therefore accessing a page table to determine a physical address corresponding to a linear address causes a memory access, which may delay processing time. In order to alleviate this concern, many processor implementations include a high speed memory or bank of registers within the processor termed a translation lookaside buffer (TLB) in which some subset of the current linear to physical memory mappings that are in use is cached, based on the values in the page table. This allows a processor to more rapidly access a translation of a linear address to the corresponding physical address than would be possible in general if the processor had to access the page table. Processor implementations generally provide instructions to manage the TLB, including an instruction to invalidate or update all the entries in the TLB based on current translations as stored in the page tab.